The invention relates to a process and apparatus for coating metal strips on both sides with an enamel base coat and, at least on one side, with an enamel top coat applied over the base coat, by continuously passing the strips in a contactless manner through furnaces with heating and cooling zones.
A process is known in which the metal strips, which are coated on both sides, preferably with a slurry for producing an enamel base coat, are continuously passed through a horizontal furnace with a preheating zone, a baking zone and a subsequent cooling zone. As a rule, an enamel base coat is applied first to both sides of the metal strip in a first furnace and an enamel top coat made of slurry or powder is subsequently applied to one side of the strip in a second baking furnace of the same type.
Due to the fact that the metal strips are horizontally guided at baking temperatures in the furnace ranging from 700.degree. to 900.degree. C., the freely suspended loop in the furnace sags considerably. Thus, the maximum length of the baking furnace is limited by the tensile load on the strip in the freely suspended loop. Furthermore, this results in different spacings relative to the burners or burner tubes, which are arranged in the furnace in the longitudinal direction of a fixed, predetermined curve of sag. As a result, a constant baking temperature and uniform enamel quality cannot always be assured at high operating speeds of the equipment. Furthermore, the inlet and outlet openings on the baking furnace have to be dimensioned relatively large, so that such furnaces have high heat losses. Moreover, the efficiency of such a plant is relatively low due to the limited length of the furnace.
Accordingly, it is an object of the invention to provide a more efficient and qualitatively enhanced process and apparatus for coating strips on both sides with an enamel base coat and at least on one side with an enamel top coat.